Electron tunneling is a quantum phenomenon in which electric current can pass from one electrode through a thin insulating barrier layer into a second electrode. This three layer system--electrode, barrier and counter-electrode--is referred to as a tunnel junction. Where the electrodes are made of ferromagnetic material, a ferromagnet-insulator-ferromagnet (FM-I-FM) trilayer tunnel junction is formed. The intervening thin insulating layer is known as "tunnel barrier" and has thickness less than about 10 nanometers. The current flowing across such a trilayer tunnel junction structure depends on the relative magnetization (M) direction of the FM electrodes. The junction resistance is higher when the magnetization of one FM is antiparallel to that of the other FM and lower when they are parallel to one another.
Julliere, Phys. Lett. 54A, 225 (1975), proposed an explanation for the change in junction resistance with change in magnetization direction. His explanation is based on the conduction electron spin polarization values of the FM electrodes, a model that later groups have essentially adopted. According to this model, if P.sub.1 and P.sub.2 are the conduction electron spin polarizations of the two FM electrodes, as measured by spin-polarized tunneling experiments with superconductors, the change in the tunnel conductance or resistance is given by: EQU .DELTA.R/R=(R.sub.a -R.sub.p)/R.sub.a =(G.sub.p -G.sub.a)/G.sub.p =2P.sub.1 P.sub.2 /(1+P.sub.1 P.sub.2) (1)
Here R.sub.p and R.sub.a are the resistances with magnetization of the electrodes parallel and antiparallel respectively and G.sub.p and G.sub.a are the equivalent conductances. For an Fe--Co tunnel junction, with polarizations of 40% and 34% respectively for the two FM electrodes, the above expression yields a 24% (.DELTA.R/R) change in the tunneling conductance between antiparallel and parallel orientation of M in the two FM electrodes.
This is an ideal case, which neglects limiting factors, such as, domain walls in the junction area, interfacial and barrier spin scattering, direct coupling between the two FM films, and surface degradation of FM films. In practice, all of these factors and perhaps others diminish the expected effect.
Reports by various groups using mainly nickel oxide (NiO) and aluminum oxide (Al.sub.2 O.sub.3) barriers between nickel (Ni) and cobalt (Co) electrodes establish the occurrence of a change in resistivity with change in magnetization direction for FM-I-FM tunneling. However, in most of these cases, the change in the tunnel resistance .DELTA.R/R was 2-6% at 4.2 K., and only fractions of a percent at room temperature. Recent experimental work by Miyazaki et al., J. Magn. Magn. Mater. 126, 430 (1993), showed a 2.7% change in the resistance at room temperature. In their experiment, part of the 150 .ANG. Al film over a permalloy film was oxidized to form NiFe/Al--Al.sub.2 O.sub.3 /Co tunnel junctions.
Scientists, for many years, have known in theory about the fundamental dynamics of the tunnel resistance arising from conduction electron spin polarization. However, the past efforts in this area have failed to produce an adequate level of change in the tunneling resistance (.DELTA.R/R) for any practical and effective use of the phenomenon. Consequently, a need exists for an FM-I-FM trilayer junction construction in which the magnitude of the junction resistive change is at least 10%. Such a junction would then find a practical use as a memory or sensor device.